One class of pressure mapping systems are based on a matrix of capacitors in which a thin, flexible elastomer is used as the dielectric. When pressure is applied to the capacitor, the dielectric material is compressed and the capacitance changes. The sensing mechanism of the sensor depends on two main factors: the mechanical properties and the geometry of the elastomer. Both of these properties determine the creep, compression set, and hysteresis of the sensor.
To generate a pressure map, an array of m rows and n columns of conductive strips are separated by a the thin compressible elastomeric dielectric. This creates an m×n array of sensor elements which are rastered to generate a map of the pressure distribution. An electrical signal is applied sequentially to each row, and the attenuated signal is detected sequentially from each column. The pressure applied to any individual sensor element will compress the elastomer, and increase the capacitance of the element, thus increasing the detected signal strength. The signal strength is correlated to a pressure value through a process of calibration. All adjacent rows and columns are switched to electrical ground to reduce interference between contiguous sensor projections. It is assumed that each sensor element is an independent entity and that an applied pressure on one sensor element does not affect the output of another sensor element.
The elastomer is conventionally a foam material, or a solid, non-foam sheet.
Foams are classified as either open or closed cell. In the open cell structure, the cells (air pockets) are interconnected while in the closed cell structure there is a predominance of non-interconnecting cells. Closed cell foams typically have higher compressive strength due to their structures and are also generally denser and heavier than open cell foams. Foams are extremely lightweight and flexible compared to non-foam dielectrics and can be very sensitive to low applied pressures.
Unfortunately, one of the largest drawbacks to foams is the inherent random nature of the cell structure. The cell structure is the source of the substantial creep and large amounts of hysteresis which is characteristic of both open and closed cell foams. The rebound of foam when pressure is decreased tends to be much slower than a solid elastomer due to the fact that air (or some other gas) needs to re-enter the foam to prevent a vacuum from foaming within the cells. Slower response time and greater compression set make it difficult to detect dynamically changing loads accurately. A sensor using foam may experience inaccuracies such as a lack of repeatability due to the inconsistency of the film, unpredictable deformation, and mechanical instability. These factors create problems when foam is used in a capacitive pressure sensor.
Solid elastomer dielectrics ameloriate some of the issues that the use of foam presents. The simplest way to employ a solid elastomer as the dielectric is via a uniform solid sheet The hardness of the elastomer will have a large impact on the physical properties of the final film. A low durometer elastomer will be softer, more flexible, and have a higher sensitivity at low pressures compared to a high durometer elastomer, but will exhibit more creep and have a lower maximum pressure limit. The chemistry of the elastomer will also affect the final physical properties such as tear resistance. When compared to foam, solid elastomers will typically have a higher density but may have comparable flexibility.
If a solid sheet elastomer is compressed under a single point load, the sheet will be compressed in the region under the applied force. Depending on the material composition, the depression can be localized or may spread radially outwards. If there are multiple compression points that are in close proximity, the deformation of the elastomer in the vicinity of each depression can be unpredictable and leads to buckling of the sheet or an increase in thickness, which is the primary problem with using a solid sheet. This is due in part because when the solid sheet is compressed, there is no empty space for the compressed material to expand into without distorting the surrounding uncompressed material.
A solid sheet is also less compressible and will reach its maximum compression quickly and will thus saturate the sensor. Therefore, solid sheets are less sensitive to changes in pressure.
The use of a patterned solid elastomer may mitigate one or more of the disadvantages of the prior art.